This patent application relates generally to digital radio telecommunications systems and, in particular, to synchronous Code Division Multiple Access (CDMA) telecommunications that employ pseudonoise (PN) spreading codes, such as Hadamard or Walsh codes.
In synchronous CDMA systems the PN codeset that is typically used to differentiate users is the Walsh set. However, the Walsh codeset has poor spectral properties and, as a result, it is desirable to provide some randomization of the codeset. It is known in the art to scramble the Walsh code by generating another PN code, i.e., a xe2x80x9ccover codexe2x80x9d, of the same length as the Walsh code, and to then exclusive-OR (XOR) each code in the Walsh codeset with the cover code. However, this technique does not always provide the best improvement in the spectral properties of the Walsh codes used by the system.
General reference in this regard can be had to U.S. Pat. No. 5,751,761, entitled xe2x80x9cSystem and Method for Orthogonal Spread Spectrum Sequence Generation in Variable Data Rate Systemsxe2x80x9d, by K. S. Gilhousen.
It is an object of this invention to provide an improved technique for providing PN spreading codes for use in a CDMA radio telecommunications system.
It is another object of this invention to improve the performance and reliability of synchronous CDMA systems.
It is a further object of this invention to improve the spectral properties of a PN codeset for use in channels of a synchronous CDMA telecommunications system.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A method in accordance with the teachings of this invention improves the spectral properties of a PN codeset, such as a Hadamard PN codeset, for use in channels of a synchronous CDMA telecommunications system. The method comprises steps of (a) providing the PN codeset matrix; and (b) reordering the columns of the PN codeset matrix in accordance with a reordering pattern. The method may further include the step of inverting at least one codeword of the reordered PN codeset in accordance with an inversion pattern, and may further include the step of selecting PN codes to optimize a subset of channels to operate in an asynchronous manner.